The present invention relates to an optical information reproducing device for reproducing a signal from an optical information medium which carries digital or the like information recorded thereon in correspondence to edge positions of marks in accordance with a mark edge recording scheme.
According to the mark edge recording scheme, digital information is recorded in the form of marks of different lengths with correspondence being established between bits xe2x80x9c1xe2x80x9d and xe2x80x9c0 (zero)xe2x80x9d on one hand and xe2x80x9cpresencexe2x80x9d and xe2x80x9cabsencexe2x80x9d of mark edges, respectively, on the other hand. A typical example of the reproducing method will be described below. When record marks are scanned therealong with a laser beam with a reflection beam being detected and converted into a reproduced signal by means of an optical detector, a reproduced signal is modulated in conformance with the record marks. By way of example, in the case of a CD-ROM disk and a phase-change disk or the like recording medium, when a light spot of the scan beam is positioned on the record mark, the reproduced signal assumes a low level, while it assumes a high level when the light spot is located in a space portion intervening between the record marks. On the other hand, when the light spot lies on an edge of the record mark, the reproduced signal assumes an intermediate level between the low and high levels. By setting a slice level at this intermediate level, an edge signal pulse is generated every time when the reproduced signal traverses the slice level. Thus, with the method described above, information recorded in conformance with the mark edge recording scheme can be reproduced.
FIG. 5 is a view for illustrating a conventional information reading method which has heretofore been known. The length of a shortest record mark 52 and that of an inter-mark space, are represented by Øm, while the length of a light spot 51 in the direction as viewed along the mark row is represented by Øs. When the center of the light spot 51 is positioned on an edge 53 of the record mark 52 and when the length Øm is shorter than a half of the diameter Øs of the light spot, detection error of the edge 53 will be occurred under the influence of an adjacent record mark. Accordingly, the minimum length Øm which allows the mark edge to be detected stably corresponds to a half of the diameter Øs of the light spot (see FIG. 5(a)).
Further, as is shown in FIG. 5(b), when the recording power is excessively large and/or in case the recording sensitivity of an information recording medium is excessively high upon information recording, the mark which is to be intrinsically recorded in a size indicated by a curve 52 may be recorded in a greater size, as indicated by a curve 54. In that case, the center axis of a read-out or reproduced signal 55 will be deviated downwardly from the slice level 56, as represented by a curve 57 in FIG. 5(c), as a result of which error will be involved in the detection of the mark edge position. When the recording power is excessively low and/or when the recording sensitivity of the information recording medium is excessively low, the center axis of the read-out or reproduced signal 55 will equally be deviated upwardly from the slice level 56, which will incur error in the detection of the mark edge position. Additionally, in the case where the reflectivity of the information recording medium changes, the center axis of the read-out signal 55 will be deviated downwardly or upwardly from the slice level 56, as indicated by the curve 57 in FIG. 5(c), which will incur a detection error of the mark edge position as well. For this reason, it is required to control the recording power with very high accuracy in order to suppress the error to a minimum. Besides, tolerance ranges of the recording sensitivity and the reflectivity of the information recording medium has to be necessarily narrow.
An object of the present invention is to provide an optical information reproducing device in which error is difficult to occur in the detection of the mark edge position even when the recording power changes or even in case the recording sensitivity or the reflectivity changes.
For achieving the object mentioned above, the present invention provides an optical information reproducing device which is comprised of a center detecting means for detecting a center of an information mark and a center between an inter-information-mark spaces, and an edge signal generating means for generating an edge signal of the above-mentioned information mark from an output signal of the above-mentioned center detecting means.